The invention relates to a method and a circuit arrangement which enable in a stepper motor a load angle of the motor to be detected in a sensor-free manner, in order to determine for example the amount of a mechanical load applied to the motor shaft (motor load-value). The invention further relates to a method and a circuit arrangement which enable the motor current of a stepper motor to be controlled in accordance with a load angle or a load value such that the current consumption of the motor is as low as possible. The invention can be used both for 2-phase and for 3- or more phase stepper motors.
Stepper motors usually are controlled and energized independent of a current, actual motor load, i.e. a required torque, in such a way that they generate sufficient torque for the highest motor load which is expected when a certain application occurs. Consequently, only two types of load scenarios are distinguished for such applications, i.e. motor standstill and motor operation.
Since the torque and the holding torque of a stepper motor for the most part are proportional to the RMS (root mean square) value of the motor current, multiplied by the sine of the load angle, the motor, in most situations, is operated with a motor current that is much too high in relation to the current actual motor load, i.e. is operated at a torque reserve that is much too high. If, for example, the motor is operated with a mean torque reserve of 100%, i.e. with a motor current that is twice as high as required, the result, given that the power loss of the motor is proportional to the square of the motor current, is a static power loss that is increased by a factor of four. Since the mechanical power taken in stepper motors often is small compared to the static power loss, energy losses ranging from 100 to 300% are to be expected, which actually is unnecessary. While a reduction of these losses would be possible by using rotary sensors (position sensors, rotary pulse generators, rotary angle sensors) and by analyzing the rotational positions of the rotor, this concept is rejected for the plurality of cost-sensitive stepper motor applications in which the stepper motor is used as a purely forward propelled drive and in which such or other rotation sensors thus are not required.
It is desirable to look for a possibility to lower the current consumption of a stepper motor especially by supplying it, based on an amount of a current, actually applied mechanical motor load or a load angle, with just enough current that is needed to provide the required torque.
It is desirable to provide a method and a circuit arrangement which enable the amount of a current mechanical load (load value) that is actually applied to the motor shaft of a stepper motor or a load angle of said motor to be detected in a sensor-free manner, in order for example to control the motor in such a way that its current consumption is as low as possible.
It is also desirable to provide a method and a circuit arrangement which enable the operation of a stepper motor based on a current, actual mechanical load (load value) applied to the stepper motor or a load angle of said motor such that the current consumption of the motor is as low as possible.
A special advantage of these solutions is that they can be realized in a relative easy manner as a component of a known stepper motor control circuit without requiring any changes to the mechanical design of the motor or without requiring any additional mechanical components such as rotary sensors, for example.